In a typical radio communications network, communication devices, also known as Mobile Stations (MSs) and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more Core Networks (CN). The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink and/or uplink cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM).
General Packet Radio Service (GPRS) is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications (GSM).
Enhanced Data rates for GSM Evolution (EDGE) also known as Enhanced GPRS (EGPRS), or IMT Single Carrier (IMT-SC), or Enhanced Data rates for Global Evolution is a digital mobile phone technology that allows improved data transmission rates as a backward-compatible extension of GSM.
The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
The project covers cellular telecommunications network technologies, including radio access, the core transport network, and service capabilities—including work on codecs, security, quality of service—and thus provides complete system specifications. The specifications also provide hooks for non-radio access to the core network, and for interworking with W-Fi networks.
In some versions of the RAN as e.g. in UMTS or GSM, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs and BSCs are typically connected to one or more core networks.
Specifications for Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and are further evolved in coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the LTE radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein radio base station nodes are directly connected to the EPC network, i.e. a radio network controller concept as realized in UMTS with a Radio Network Controller (RNC) does not exist. In general, in EPS the functions of an RNC are distributed between eNBs and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio base stations without being controlled by RNCs.
Machine Type Communications (MTC) is an area within telecommunications, sometimes also referred to as M2M or Internet of Things (IoT), in which it is envisioned that all types of devices which may potentially benefit from communicating will do so. That is, everything from agriculture and/or industrial sensors and actuators to things in the smart home or workout gauges in the personal networks will be connected wirelessly.
MTC has in recent years shown to be a growing market segment for cellular technologies, especially for GSM and Enhanced Data Rates for GSM Evolution (EDGE) with its global coverage, ubiquitous connectivity and price competitive devices.
With more and more diverse MTC applications, more and more diverse set of MTC requirements arise. Among these there is a low-end market segment characterized by some or all of the following requirements compared with the current GSM technology:                Extended radio coverage        Long battery life        Low device complexity        Large number of connected devices        
Today's cellular systems are not always suitable for new applications and devices that follow with MTC and Internet of Things (IoT). For example, there is an objective to increase a radio coverage compared to existing services. In the following, the use of the word coverage will refer to radio coverage. In telecommunications, the coverage of a base station is the geographic area where the base station is able to communicate with wireless devices. Some MTC networks are envisioned to be deployed in extreme coverage circumstances, such as basements of buildings or beneath the ground where radio signals suffer from severe attenuation.
At a 3GPP meeting GERAN#67 a new work item called ‘New Work Item on Extended Coverage GSM (EC-GSM) for support of Cellular Internet of Things’ was approved with the intention to improve coverage with 20 dB, to improve battery life time and to decrease device complexity. Later the name EC-GSM was changed to Extended-Coverage Global System for Mobile communications Internet of Things (EC-GSM-IoT), and these two names will be used interchangeably hereafter.
Cellular Internet of Things' provides IoT by means of a cellular system, such as EC-GSM-IoT.
An extended coverage, e.g. a coverage range beyond that of legacy GPRS/EGPRS operation may be achieved by blind physical layer repetitions in both uplink and downlink. The number of repetitions may be associated to a given Coverage Class (CC).
Logical channels supporting operation in extended coverage are referred to as Extended Coverage channels (EC-channels). On a control channel, i.e. on an EC control channel, the coverage may be improved using blind physical layer repetitions of radio blocks while on a data channel, i.e. on an EC data channel, the coverage may be improved using a combination of blind physical layer repetitions and HARQ retransmissions of radio blocks. “Blind Physical Layer Repetitions” means that a predetermined number of repetitions are sent blindly, i.e. without feedback from the receiving end.
Taking the example of EC-GSM four different Coverage Classes are defined denoted as CC1, CC2, CC3 and CC4 respectively. Each Coverage Class is approximated with a level of extended coverage range compared to legacy GPRS/EGPRS operation. I.e. each Coverage Class represents a certain amount of degradation of a signal over noise ratio compared to legacy GPRS/EGPRS operation, e.g. 3 dB, such that the number of blind physical layer repetitions associated with each Coverage Class is proportional to its corresponding degradation compared to legacy GPRS/EGPRS operation. For example, for the EC Packet Data Traffic CHannel (EC-PDTCH) CC1 corresponds to one single transmission, CC2 corresponds to 4 transmissions, i.e. 3 repetitions, CC3 corresponds to 8 transmissions and CC4 corresponds to 16 transmissions. Thus, CC1 corresponds to the coverage range of legacy GPRS/EGPRS operation, i.e. extended coverage not used.
Further, in EC-GSM-IoT a fixed predefined number of blind physical layer repetitions are applied per logical channel. The number of blind physical layer repetitions may differ between logical channels for the same Coverage Class.
The approach of blind physical layer repetitions on the EC-channels will result in a decrease in the data rates and thus longer latencies compared to the legacy GPRS/EGPRS operation for sending and receiving messages between the network, such as the core network, and the mobile stations. Non Access Stratum (NAS) messages are messages that are sent transparently via the radio access network between the mobile station and the core network, e.g. a Serving GPRS Support Node (SGSN). The NAS messages are supervised by timers defined in 3GPP TS 24.008 v13.3.0 Technical Specification Group Core Network and Terminals; Mobile radio interface Layer 3 specification; Core network protocols; Stage 3.
When e.g. a mobile station enters a cell which belongs to a new Routing Area (RA), an RA Updating (RAU) procedure is started by the mobile station, whereby the mobile station sends a ROUTING AREA UPDATE REQUEST message to the SGSN and starts a NAS timer T3330. The timer has a defined value of 15 seconds in 3GPP TS 24.008 V13.3.0, and is normally stopped at reception of the ROUTING AREA UPDATE ACCEPT message or at reception of the ROUTING AREA UPDATE REJECT message sent from SGSN. At expiry of T3330, i.e. when neither ROUTING AREA UPDATE ACCEPT nor ROUTING AREA UPDATE REJECT message has been received, the procedure is started again, at most four times. At the fifth expiry of timer T3330, the mobile station shall abort the procedure.
During the RA updating procedure the SGSN may trigger the authentication and ciphering procedure whereby the SGSN sends the AUTHENTICATION AND CIPHERING REQUEST message to the mobile station and starts a NAS timer T3360. The timer has a defined value of 6 seconds in 3GPP TS 24.008 V13.3.0, and is normally stopped at reception of the AUTHENTICATION AND CIPHERING RESPONSE message or at reception of the AUTHENTICATION AND CIPHERING FAILURE message sent from the mobile station. At expiry of T3360, i.e. when neither AUTHENTICATION AND CIPHERING RESPONSE nor AUTHENTICATION AND CIPHERING FAILURE message has been received, the AUTHENTICATION AND CIPHERING REQUEST message is retransmitted again and the T3360 timer is reset and restarted, at most four times. At the fifth expiry of timer T3360, the SGSN aborts the procedure.
The mobile station will have to restart the RAU procedure after each timeout. This implies that more power will be consumed by the mobile station which is a problem for battery limited devices. This is particularly a problem for mobile station operating in extended coverage, for example due to the requirement of long battery lifetime.
If the mobile station has tried the RAU procedure the maximum number of times allowed and failed each time, then the mobile station goes out of service, potentially for long time periods.